The present invention relates to optical surface preparation techniques, and more particularly, to improving the capability of the surface of a structure to bond with a material or the surface of another structure by irradiating one or both surfaces with incoherent, pulsed optical energy having a broadband energy spectrum.
There are many applications for which it is desirable to bond one material to another. In the automotive industry, for example, polymeric outer body panels are often bonded to metal subpanels to improve aesthetics. In the defense industry, composite structures bonded to each other are used routinely for weight reduction. In the aircraft industry, the exterior surface of airplanes must be painted to prevent corrosion that could otherwise weaken the airplane structure. Good bond strength between materials depends to a large extent on appropriate surface preparation. Surface preparation techniques which enhance the capability of the surface of a structure to bond with a material (bondability) include solvent cleaning, abrading, and/or chemical treatment. However, these methods are all characterized by potential toxic chemical hazards, waste disposal problems, and high production costs.
All surface preparation techniques generally increase the surface free energy of the surface. Surface free energy refers to the energy required to create a unit area of the surface. In the case of a liquid, the surface free energy is expressed by the surface tension coefficient. Polar liquids, such as water, have high surface tension coefficients (H.sub.2 0=73 dynes/cm at 20.degree. C.). For a liquid to wet a solid surface, the surface free energy of the solid surface must therefore be higher than that of the liquid. As a result, the ability to achieve a water break free surface (i.e., no beading) is often used as a criteria for adequate surface preparation.
Surface free energy of a solid surface can be improved by either removing all surface contaminants or by changing the surface chemistry. Removing all surface contaminants improves the surface free energy because, when exposed to the environment, a solid with inherently high surface free energy will attract contaminants as a way to reduce its total energy. As a result, the contaminated surface loses or reduces its ability to bond to other surfaces. Removal of the surface contaminants (which are typically organic materials) will restore the surface's inherent surface free energy. For example, metals such as aluminum can achieve a "water break free" condition when the surfaces are clean.
Removing surface contaminants does not always help, however, depending upon the type of material. Most organic materials (particularly those made up of large chains of molecules), for example, usually have low surface free energies regardless of the cleanliness of the surfaces. Hence, to increase the surface free energy, the surface molecular structures of such materials must be modified. For example, the surface affinity for other molecules can be increased by either breaking up the large molecular chains into smaller ones or by the insertion of other atoms into molecular chains at the surface.
Optically engineered surface preparation technology is a known alternative to solvent, abrasive, or chemical processes, and avoids some of the aforementioned problems. An example of one optical surface preparation technique is presented in Sowell, R. R., et al., "Surface Cleaning By Ultraviolet Radiation," J. Vac. Sci., Vol. 11, No 1, January/February 1974. The Sowell reference describes a process for removing hydrocarbon contaminants from metal and glass surfaces by irradiating such surfaces with generally steady-state Ultra-Violet (UV) radiation in the presence of a low pressure oxygen atmosphere or in open air. However, the process described by Sowell requires hours to complete due to the limited UV light intensity that can be obtained from a steady-state UV light source. Therefore, the Sowell process is generally not suitable for applications in which it is desirable to increase the bondability of surfaces, some of which may be very large, within a period of time which would make such processing practical.
U.S. Pat. No. 4,803,021, "Ultraviolet Laser Treating of Molded Surfaces," is directed to a method for preparing the surfaces of molded products to improve bonding and painting performance. Such method includes irradiating the coated surface of molded products with pulsed laser light that decomposes any mold-release agents present on the surface to yield diverse decomposition fragments within the irradiated zone. This process requires that the surface material be etched deeply enough to remove substantially all of the mold-release agent. A surface may only be subjected to this process a finite number of times in order to limit the amount of surface material removed by etching. Since molded plastic is generally released from a mold only once, the minimal amount of material removed by etching is tolerable and may be considered in the design of the such products. However, this process is not suitable for repeatedly treating surfaces as part of a scheduled maintenance program, as for example, where it is desired to prepare a surface for painting, if preservation of the surface is desired.
A significant problem with a laser based system, such as that described by the '021 patent, is that irradiation of large or topologically complex surfaces with the pinpoint beam of a laser is very difficult to achieve, requiring sophisticated scanning and rastering techniques. Furthermore, the operation of a laser requires laser stops to prevent the laser beam from inadvertently escaping the work area, and the building where the laser is operated. This is because lasers pose a serious danger to humans, who could be seriously injured if irradiated with a laser beam.
An even more significant problem with laser illumination, however, is controlling the intensity with which a laser beam irradiates a surface. Because the intensity of a laser beam does not follow the inverse square law, the energy density of the area or "footprint" irradiated by the laser beam is generally so high that the beam causes thermal decomposition of the materials at the surface being irradiated. If the structure being irradiated is made of a polymeric material, laser irradiation breaks up the thermal bonds of the molecules of the material, but then (due to the excessive influx of energy) causes the decomposed polymer bonds to recombine into new, and different polymer molecules. The formation of such new polymer molecules may actually cause a decrease in the surface free energy of the irradiated surface, causing the bondability of the surface to possibly decrease.
Thus, it may be appreciated that there is a need for a process which enhances the capability of the surface of one structure to be bonded to another. A further need exists for a cost effective method to increase the bondability of large or topologically complex surface areas. A still further need exists for a method to increase the bondability of a surface which does not require the use of toxic chemicals.